Fabrication and characterization of ZnO/p-Si light-emitting devices by electron beam evaporation

ZnO is a direct and wide band-gap (Eg=3.37 eV) semiconductor with a large exciton binding energy (60 meV) and wurtzite crystalline structure under standard conditions. It is a non-toxic, earth-abundant, low-cost material... [ view full abstract ]

ZnO is a direct and wide band-gap (Eg=3.37 eV) semiconductor with a large exciton binding energy (60 meV) and wurtzite crystalline structure under standard conditions. It is a non-toxic, earth-abundant, low-cost material compatible with silicon technology and transparent in the visible range, with tunable electrical conductivity depending on doping and post-annealing processes. It allows for a high-temperature and high-power operation, which gives this material a low noise generation and makes it suitable to sustain large electric fields.
In this work we focused on the fabrication of light-emitting devices of ZnO thin films by electron beam evaporation, deposited on p-type silicon substrates [see Fig.1(a)]. ZnO thin films were fabricated using electron beam evaporation technique and subsequently annealed at 750 ºC for 1 hour in either N2 or air. Several characterization techniques were employed to determine their properties, such as SEM, XRD, Raman spectroscopy and PL. The electro-optical behavior of the devices was analyzed using optimized ZnO films by a probe station coupled to a monochromator.
SEM images of the films displayed a textured surface with clear contrast in the grain boundaries, revealing that the thin films present a polycrystalline arrangement. XRD confirmed also that thin layers were highly oriented in the (002) direction and with a mean grain size of 40 nm. Raman scattering measurements revealed a good crystalline degree for all annealed samples. PL measurements exhibited optical emissions originated from defects and band-to-band exciton recombination processess.
ZnO-based devices with a metal-oxide-semiconductor structure were fabricated from previous films by adding ITO and Al as top and bottom contacts, respectively. The devices present I-V curves that follow an exponential trend. Electrical measurements revealed higher conductivity in samples annealed in N2, attributed to a higher density of oxygen vacancies. The integrated electroluminescence exhibits an almost linear behavior with the injected current, which saturates at high injection regime [see Fig.1(b) and 1(c)]. Spectrally-resolved electroluminescence showed an emission related to defect states within the band-gap [see Fig.1(d) and 1(e)]. Finally, a photometric study revealed a CRI of 91.8 for our final devices. Thus, ZnO-Si heterostructures show potential to be used as an integrated light emitter.